The present invention relates generally to material transport systems and more particularly to conveyor belt systems.
In many conveyor belt applications it is desirable to measure or control the weight of material moving past a first reference point on a belt, which material is delivered downstream to a second reference point on the belt, usually the discharge end. Consequently, substantial effort has been expended to develop accurate conveyor belt scales which utilize an input sensor at the first reference point for producing an output signal related to the weight of material on an associated portion of the belt. Examples of such scales are the Thayer Series RF Belt Scales manufactured by Hyer Industries, Inc., the assignee of the present invention. Scales of this type permit in-motion weighing for weight totalizing and flow rate control in material transport systems. In many applications such scales are used in a system incorporating an electronic integrator which receives weight signals from a belt scale and a belt speed signal from the conveyor belt drive means. The integrator integrates the product of these two signals and provides an output signal which is indicative of the weight of material that passes on the portion of the belt associated with the scale input sensor. Electronic integrators of this type are well known in the art, and may be of the form shown in U.S. Pat. No. 3,610,908 to Raymond Karosas, dated Oct. 5, 1971 and assigned to the assignee of the present invention.
U.S. Pat. No. 3,559,451 to Frank S. Hyer and Raymond Karosas, dated Feb. 2, 1971, and assigned to the assignee of the present invention, describes a totalizing and flow rate measuring system which includes an integrator of the type noted above to generate a digital weight signal which is subsequently processed to produce output signals or indications representative of the cumulative weight and the instantaneous flow rate of material on the belt which passes the input sensor of the scale.
In the conveyor belt material transport systems known in the art, as described above, a substantial problem may arise from the choice of the belt scale input sensor location between the tail and discharge ends of the belt.
This problem results from the conflicting requirements of in-motion weight measurement on the one hand, and of providing accurate data on material delivery or correlation with external processes on the other hand. From the standpoint of in-motion weight measurement, the optimum location of the scale input sensor for accurate measurement is at the point of least belt tension in the conveyor, allowing for the material to settle in stable form on the belt prior to reaching the scale input sensor. This requirement dictates the placement of the scale input sensor near the tail end of the conveyor belt. With the scale input sensor so located, any cumulative weight measurement or flow rate measurement is related to a position remote from the discharge end. Therefore, such a system does not reflect the quantity of material on the belt portion between the sensor and the discharge end or variations in the material loading along that portion of the belt. The resulting inaccuracies are referred to herein as problems of transport lag. The material being delivered by the conveyor transport system to an external process or to a receiving container (e.g., a truck or railroad car) is that which is discharged from the head pulley or discharge end. Therefore, from the standpoint of accuracy in material delivery or external process, the optimal position for the scale input sensor would be at the discharge end. However, in certain applications, when the input sensor of a belt scale is positioned near the relatively high tension discharge end of a conveyor belt, the accuracy of the scale is substantially impaired.
In the prior art, the usual practice has been to position the scale input sensor near the lower tension tail end whenever the input weighing accuracy is of primary importance (e.g., plus or minus one-half percent or better), to position the scale near the higher tension discharge end (and bearing with the resulting scale inaccuracies) when the output weighing accuracy is of primary importance, and to position the scale at some intermediate point selected to compromise between the conflicting requirements when they are of more nearly equal importance. In implementing such a tradeoff, the loss in performance accuracy due to both effects is a substantial drawback in certain prior art conveyor belt material transport systems.
A further difficulty arises in certain prior art systems where the configuration of the conveyor belt does not permit the positioning of a scale input sensor in an appropriate position relative to the belt. In such systems s weigh feeder, such as the Thayer Series MXL weigh feeder manufactured by Hyer Industries, Inc., may be used to deliver the material to the conveyor belt. A weigh feeder typically comprises a belt scale and a relatively short conveyor belt having substantially none of the scale accuracy and transport lag problems associated with the relatively long main conveyor belt. The drive means of the weigh feeder belt are driven by a demand weight signal which represents the desired weight of material to be fed to the main belt from the discharge end of the weigh feeder belt. However, even when the weight of material added to the main belt in this manner is accurately measured and controlled by the weigh feeder, the system may have inaccuracies due to transport lag as will be clear from the following description. These difficulties are a consequence of the inability to identify the precise time at which each portion of material added by the weigh feeder has reached a particular downstream point on the main belt.